Semiconductor devices include insulating films made of SiO2, PSG (Phospho Silicate Glass), P-SiO (“P” stands for formation by plasma CVD), P-SiN (“P” stands for formation by plasma CVD), SOG (Spin On Glass), Si3N4 (silicon nitride), etc. As a method of forming such a silicon oxide film or silicon nitride film on the surface of a semiconductor wafer, there is known a method of forming a film by thermal CVD (Chemical Vapor Deposition), which employs a silane family gas, such as monosilane (SiH4), dichlorosilane (DCS: SiH2Cl2), hexa-chlorodisilane (HCD: Si2Cl6), bistertialbutylamino-silane (BTBAS: SiH2(NH(C4H9)2), as a silicon source gas.
Specifically, for example, where a silicon oxide film is deposited, the thermal CVD for forming the silicon oxide film is performed, using a gas combination, such as SiH4+N2O, SiH2Cl2+N2O, or TEOS (tetraethyl-orthosilicate)+O2. Where a silicon nitride film is deposited, the thermal CVD for forming the silicon nitride film is performed, using a gas combination, such as SiH2Cl2+NH3, or Si2Cl6+NH3.
Owing to the demands of increased miniaturization and integration of semiconductor devices, insulating films such as those described above need to be made thinner. Furthermore, in order to maintain the electric properties of the various films that lay below insulating films, the temperature used in thermal CVD in forming the insulating films needs to be lowered. In this respect, for example, where a silicon nitride film is deposited, thermal CVD for forming the silicon nitiride film is conventionally performed at a high temperature of about 760° C. In recent years, thermal CVD for forming the silicon nitiride film is performed at a lower temperature of about 600° C., as the case may be.
When semiconductor devices are fabricated, a conductive film and an insulating film as described above are stacked and pattern-etched to form a multi-layer structure. Where an insulating film is formed and another thin film is then formed thereon, contaminants such as organic substances and particles may have stuck to the surface of the insulating film. In order to remove the contaminants, a cleaning process is performed, as needed. In this case, the semiconductor wafer is immersed in a cleaning solution, such as dilute hydrofluoric acid, to etch the surface of the insulating film. By doing so, the surface of the insulating film is etched by a very small amount, thereby removing the contaminants.
Where an insulating film is formed by CVD at a high temperature of, e.g., about 760° C., the etching rate of the insulating film during cleaning is very small. Accordingly, the insulating film is not excessively etched by cleaning, and thus the cleaning process is performed with a high controllability in the film thickness. On the other hand, where an insulating film is formed by CVD at a low temperature of, e.g., about 600° C., the etching rate of the insulating film during cleaning is relatively large. Accordingly, the insulating film may be excessively etched by cleaning, and thus the cleaning process entails less controllability in the film thickness.